Direct reading meter



April 13, 1943.

G. H. BROWN DIRECT READING METER (Ittoineg Patented Apr. 13, 1.943

DIRECT READING METER- George H. Brown, Haddoniicid, N. 1., assignor to Radio Corporation oi America, a corporation of Delaware Original application June 29, 1940, Serial No. 343,082. Divided and this application March 28, 1942, Serial No. 436,591

7 Claims.

This application is a division of my application Ser. No. 343,082, filed June 29, 1940.

This invention relates to direct reading meters and especially to a meter for reading directly power, resistance or reactance in high or low frequency electrical circuits.

Wattmeters, for the measurement of electrical power, are well known to those skilled in the electrical art. Power in a load is equal to the square of the current through theload times the resistance of the load (W= I R). It follows that the resistance of the load equals the power in the load divided by the current squared or, R='W/I (1). If the current through the load is held at a predetermined value, the wattmeter may be calibrated to indicate directly resistance in ohms.

Wattmeters employing resistors, thermocouples, and a meter for reading power as a function of the currents generated by the thermocouples have been designed. These prior devices employed circuit arrangements which produced meter deflections proportional to the scalar product of current and voltage together with a term proportional to the square of the load voltage, or a term proportional to the square of the load current, or both.

One of the principal objects of the present invention is to provide means whereby these undesired terms are eliminated from the wattmeter readings. Another object is to provide means for reading directly power in a load circuit without the introduction of undesired terms, which are functions of load voltage or load current. 'Another object isto provide means for determining directly the resistance of a load circuit as a function of the power applied to the load. An additional object is to provide means for determining the reactance of a load circuit as a function of the power applied to the load.

The invention will be described by referring to the accompanying drawing in which Figure 1 is a schematic circuit diagram of one embodiment of the invention, andFigure 2 is a circuit diaram of a meter circuit which is balanced with respect to ground. Similar reference letters are applied to similar elements in the two figures.

Referring to Fig. 1, a pair of terminals A, B are connected to a power source such as an alternator or oscillator E. A pair of resistors R3, R4 are serially connected across the terminals A, B. A capacitor C or an inductor L, preferably the vformer, may be substituted for the resistor R4 by a. switch F. A pair of resistors R1, R: and the heater elements G, H of a pair of vacuum thermocouples J, M are connected in series across the resistor Ra. The thermocouple output 'elements N, P are connected in series opposition to the output meter Q. The load circuit Rt-HXL, plus the load meter 8, is connected from the Junction T, or point of symmetry, of the thermocouple heaters to the lower terminal U, or ground, of the resistor R4, which may be grounded. The thermocouples J, M, should be perfectly matched. If they are not matched. the thermocouple with the larger output voltage may be shunted by a resistor Y. When the thermocouples are perfectly matched and equal currents are flowing through the heaters G, H, the output'meter Q deflection will be zero.

Before considering the operation of the circuit, the mathematical theory and proof or operation will be deduced. The several resistances of the several resistors are designed as R1, R2, R3 and R4. The output load as Rr-HXz: the voltage across the output load as v, and the current through the load as t. Then =izR:+t4R4 (2) and, where 0 represents zero,

- 0=-tiRi+izRz+I'sR: (3) Also,

=i1 (4) and i4=ia-iz=t:t+ii (5) Substituting (4) and (5) in (2) and (3), we

Substituting (10) and (11) in (4), we obtain Dil='-DRS+MI (14) In order to make the coeillcients of the terms in (11) and (14) identical, let

Then (10) becomes Let us suppose that the load impedance is The square of the absolute value of (21) is i RaRL RrXz. 2R3RL 2:... (In!) 4 R R4 R1134) l l] (23) and (22) yields i RgR 2 R3R1, 2123124,]

2:... 2]) 4 [1+( RIB (2 The output meter Q of the pair of thermocouples reads proportional to the difference of the q tities given by (23) and (24) This meter reading M is Where K is the thermocouple constant. That is, if only one heater carries current, this current squared multiplied by K would give the meter deflection. If now the current in the load is held constant, by adjustment of the source E, the deflection oi the output meter Q will be exactly proportional to the load resistance, even through the load impedance has a reactive component.

By way of example, the following elements were employed: Each thermocouple had a maximum rated heater current of 0.10 ampere, a heater resistance of 2.3 ohms, and a constant K equal to 67,500 (it the heater current is expressed in amperes and the output meter deflection is microamperes). The thermocouple output meter Q had a full scale deflection of 200 microamperes and a resistance of 8.0 ohms.- R1, R3 and R4 were so chosen that when the load current i was held at 0.075 ampere (75 milliamperes), the thermocouple output meter directly indicated the load resistance. The load current i having been chosen, R1 and R: were selected. Then R4 was made to have the value which made the load meter S read properly. R2 was adjusted to the value indicated by Equation 16. This is done by shortcircuiting the load impedance, feeding a small amount of power into the remaining network, and adjusting R: until the reading of the thermocouple output meter Q was zero.

Since a thermogalvanometer type meter with a full scale deflection of 120 milliamperes is used in serie with the load impedance to indicate the proper load current, the meter will always read the sum of the load resistance and the meter resistance, which is 5.2 ,ohms. In further discussion, this meter resistance will be considered as part of the load resistance.

By way 01' example, the speciflc values used in a practical instrument are as follows, it the infstrument is to read a range or resistances from zero to 20 ohms: With a load resistance of 20 ohms, the output meter should read 200, which is iull scale. R1 is made equal to 20.0 ohms. This M=67,500X0.075'X10X20 (26) It is important to examine the currents which will exist in the separate thermocouple elements under various load conditions to avoid burning out the thermocouple heaters. In Table I, below, are listed the currents as a function of load resistance, when the load has zero reactance (Xz.=).

Table I R (ohms) x" i,

The greatest strain on one thermocouple comes when the load resistance is 20.0 ohms. The current conditions, when the load is reactive, with a resistance of 20.0 ohms will be examined. These results are shown in Table II, below. The reactance may be either positive or negative.

Table II R,.=2o.o)

X1. (ohms) l i l i;

Ampere: Ampere:

Other pertinent data for the condition of zero load reactance are given in the following table.

Table III L iaput Rina! Ampem Ohms 20.-. 0.1602 16.48 0.423 15. 28 0. 308

' ance.

If R4 is changed to 1895 ohms, and R2 adjusted to a value of 9.95 ohms, full scale meter reading with a load resistance of 2000 ohms will result. For this connection, the conditions existing at the input terminals are shown in Table V.

Table V R. (ohms) is... 12...... mi};

A mperea Oh ma 0. 154 984. 0 2:2. 3 0. 1345 x45. 0 15. at 0.1148 062. 0 s. 71 0. 0948 405. 0 av 63 0. 083 190. 0 1. iii 0. 0751 10. 0 0. 0563 The input power for a load resistance of 2000 ohms gets to be rather large. Fortunately, one seldom meets an antenna resistance above 500 ohms. For resistances above 500 ohms, simply set the load current to 0.70'l 0.075 or 0.053 ampere. and multiply the resulting output meter reading by two. If this expedient is followed, the maximum input power would be 11.65 watts.

From the foregoing it may be seen that the load resistance is indicated directly on an output meter Q with three ranges of resistance, namely, 0-20 ohms, 0-200 ohms, and 0-2000 ohms. It is also desirable to be able to measure the load reactance by the same means, but it is not necessary in general that the reactance be measured with the same degree of accuracy as the resist- To see how this result may be accomplished, use the meter Q on the 0-200 resistance range, with R. equal to 9.5 ohms. Substitute for R4 a capacitor C which is substantially free of resistance. The balance condition given by (16) will be upset somewhat, but will be ignored for the present. Hereinafter it will be shown that it is not of primary importance in making this re-' actance measurement.

Now assume that the system remains substantially in balance. In Equations 21 and 22 change R4 to (MC, whereC is the capacitance which is inserted, and u is 2.1. .Then the formula corresponding to (23) is Subtract (28) from (2'7), and write the equation corresponding to (25) as follows:

' :11 K'FfiwCXL If the load reactance is inductive, the output meter will read backwards, and must be reversed from the connection used in measuring resistance. If the load reactance is capacitive, the meter will read up in the same direction as when reading resistance. The deflection is proportional to the reactance of the condenser'C, which changes with the frequency. Therefore, to obtain true reactance readings, it is necessary to divide by a constant times the frequency.

11', for R4, a capacitor C is substituted which has a reactance of 189.5 ohms at a frequency of one megacycle, the meter reading is divided by the frequency, expressed in megacycles, to obtain the load reactance.

If an inductance L is substituted instead of a capacitor then an inductive load would make the meter read in the same direction as when reading resistance, and the meter reading should be multiplied by a constant times the frequency to obtain the load reactance. ln practice, inductance L is not ordinarily used.

Returning now to the eflects of the omission of reactance in the resistance R2, in order to satisfy (16), the following table has been computed from the exact expressions to show that reasonably accurate resultsare obtained notwithstandng the fact that the conditions of (16) have only been fulfilled for the corresponding resistance-measuring circuit.

Race. 1 gallon! (27 Table VI (}=l.0 mo.)

' Meter readings XL (ohms) (RL=1)0) (EL-0) To summarize the conditions existing for the various circuit elements, reference is made to the following table.

The circuit of Fig. 1 is not balanced with respect to ground; in some uses a balanced circuit is desirable. Fig. 2 is a schematic circuit diagram of a symmetrical or balanced circuit. Pairs of resistor networks R1, Ra, R3, R4 and R'i, Rz, R's, R; are used. -In a like manner, two pairs of thermocouples J, M and JM' are used. A single thermocouple output meter Q is employed. The thermocouples should either be balanced or balancing resistors (see Y of Fig. 1). In other respects, the balanced circuit is similar to that of Fig. 1.

Thus the invention has been described as a wattmeter which may be arranged to indicate directly the resistance or reactance of a load. The mode of operation may be summarized as follows:

The oscillator or power source E is first set to the desired frequency. With the proper values of R1, R3, and R4 in place, the load impedance, including the radio frequency meter S for reading load current, is shorted out. R: is then ad- Justed until the output meter Q reads zero. The short circuit on the output is then removed, the oscillator output increased until the load current is 0.075 ampere, and the output meter reading noted. This reading is the load resistance plus the radio frequency meter S resistance of 5.2 ohms. To read reactance, R4 is removed and the appropriate C inserted. The proper resistance R2 must be in place. The oscillator output is again increased until the proper load current is obtained, and the output meter reading noted. This meter reading is then divided by the frequency to obtain the reactance.

The reactance and resistance measurements may be made accurately up to several megacycles; the resistance measurements may be made accurately with direct current or sixty-cycle current from E without circuit modification.

The adjustments of C and R4 are preferably made, in practice, when the instrument is built, and do not ordinarily have to be varied during the process of measuring. While these elements have been shown as variable, in actual practice provisions are made for switching in different predetermined values of R4 and C, as will be obvious from the above description of the system.

I claim as my invention:

1. The method of measuring the reactance of a load which comprises the steps of passing an alternating current of predetermined magnitude and frequency through two separate paths to said load, controlling the division of said current between said paths in accordance with the reactance of a reactive element of known magnitude, and indicating the value of the reactance or said load as a function of the difference of the squares of the currents flowing in said paths.

2. The method of measuring the resistance of a load which comprises the steps of passing a current of a predetermined magnitude through two separate paths to said load, controlling the division oi said current between said paths in ac- 5 cordance with the ratios of resistances of known values to the resistance of said load, and producinl an indication of the resistance of said load as a function of the diil'erence of the squares of the currents flowing in said paths.

3. The method of measuring the magnitude of an impedance component of a load which comprises the steps of passing a current of predetermined magnitude and frequency through two separate paths to said load, controlling the division of said current between said paths in accordance with the magnitude of an impedance element of known value, and producing an indication of the magnitude of said impedance component as a function of the difference of the squares of the currents flowing in said paths.

4. The method of measuring power dissipated in a load which comprises the steps of dividing the current to said load between two paths in accordance with the ratios of resistances of known values with respect to the impedance of said load, and producing an indication of said power as a function 01 the diiference oi the squares of the currents flowing in said two paths.

5. The method of measuring an impedance 3o component of a load which comprises the steps of passing an alternating current through two separate paths to said load, controlling the division of said current between said paths in accordance with the ratios of impedance elements 35 of known values to the impedance of said load,

and producing an indication in terms of the difference of the squares of the currents flowing in said paths according to the equation as R B in which M represents the quantitative value of the indication, K is a constant, 1 is the load current, R1, R1 and B are the numerical values of said impedance elements, and L is the unknown impedance component of said load.

6. The method of measuring the reactance of a load which includes applying a predetermined current to said load, measuring the power thus applied to said load, and converting the power measurement by applying a correcting factor equal to the frequency of the applied current.

7. The method of measuring the reactance of a load which includes applying a predetermined current to said load, measuring the power thus applied to said load, and multiplying or dividing said power measurement by the frequency of the applied power according to whether the load is positively or negatively reactive.

GEORGE H. BROWN. 

